Typical acoustic logging tools may include, by way of example, a televiewer which comprises a rotating ultrasonic acoustic transducer that operates in a frequency range on the order of 100 kHz or more. Higher acoustic frequencies are preferred in order to achieve better resolution in the confined space of a borehole. In operation, the televiewer rotates at a desired rate such as 5 to 16 rotations per second to continuously scan the borehole sidewall as the televiewer is drawn up the borehole at a rate that is typically 3/16 to ⅜ inch per scan. A beam of acoustic pulses is launched along the normal to the borehole sidewall as the transducer scans the interior surface of the borehole. The pulse rate depends upon the desired spatial resolution such as 1500 pulses per second or 128 to 256 pulses per scan. The insonified borehole sidewall returns pulses reflected therefrom, back to the transducer on a time-multiplexed basis. The reflected acoustic signals are detected, amplified and displayed to provide a continuous picture of the texture and structure of the borehole sidewall. Other application include determination of the goodness of a cement bond to a steel casing as well as monitoring the integrity of the casing itself.
The diameter of a borehole logger is on the order of 2% in (7.3 cm), so that it can be run into relatively small boreholes. However many borehole diameters are on the order of 10-14″ (25.4-35.6 cm) or more so that the length of the acoustic-pulse trajectory from the transducer, through the borehole fluid to the borehole sidewall, may be up to 10″ (25.4 cm). In the normal course of events, the borehole fluid is contaminated by drill cuttings, air bubbles and foreign matter which severely attenuate the acoustic energy by scattering because the physical dimensions of the contaminants are comparable to the wavelength of the wavefields emitted by the transducer.
The televiewer signal is also contaminated by stationary noise. The stationary noise is due to the transducer ringing and the reverberations between the face of the transducer and a window on the outside of the televiewer assembly. The echo signal is the signal returned from the borehole formation or casing. Neglecting borehole fluid attenuation, deflection, diffraction, and window insertion loss, the amplitude of the echo signal is a function of the acoustic impedance of the formation or casing. The arrival time of the echo signal within the process window will change with the instruments centralization and the shape of the borehole. As the echo signal moves in time, it is modulated by the stationary noise, therefore the detected peak amplitude is a function of the formation reflection, and the position of the echo within the process window. If the S/N ratio is low enough, the modulation causes the resulting televiewer amplitude image to be dominated by the periodic “wood grain” interference pattern. A physical explanation of the “wood grain” is that the transducer ringing and reverberation noise is initiated when a pulse-echo signal is transmitted, so that the noise is stationary with respect to the echo signal. Strictly speaking, the term “ringdown” refers to the ringing of the transducer when it is activated. Reverberation refers to the sound that reflects back and forth between the transducer and window. Interference between the ringdown and reverberation noise and the echo signals results in a “wood grain” pattern on the amplitude image. The present disclosure addresses the problem of reducing the “wood grain” effects by improvements to the televiewer assembly, detecting the signal independently of the noise in real time, and/or filtering the stationary noise post acquisition.